Tire size calculator
Type the three numbers from the sidewall of your tyre – width, aspect ratio and rim size – to get its overall diameter and how far it rolls. Switch on the comparison to see how much a different size changes the diameter and what the speedometer will show.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Read the size on the tyre sidewall, for example 225/45R17, and enter the width, aspect ratio and rim size.
- Read the diameter, circumference and revolutions per kilometre or mile.
- To check a replacement, leave the comparison on and enter the new size.
How it works
- Sidewall = width × aspect ratio ÷ 100.
- Diameter = rim × 25.4 + 2 × sidewall.
- Circumference = π × diameter; revolutions per km = 1,000,000 ÷ circumference in mm; per mile the same with 1,609,344 mm.
- Difference = new diameter ÷ old diameter − 1. If the speedometer is set for the old size, the true speed when it shows 100 is 100 × new ÷ old.
Worked example
225/45R17 has a 101.25 mm sidewall and a diameter of 634.3 mm (24.97 in). It turns about 501.8 times per km. A 225/40R18 is 637.2 mm, 0.46% bigger, so with a speedometer calibrated for the old tyre a reading of 100 means a true 100.46. A 205/55R16 at 631.9 mm is 0.38% smaller.
Tips
- Plus-one sizing. A bigger rim with a lower sidewall can keep the diameter almost the same.
- Check clearance and load rating. Diameter is only one of the limits; the approved sizes are in your owner’s manual.
- Tread depth changes it too. A worn tyre is slightly smaller than a new one.
Frequently asked questions
What do the numbers in 225/45R17 mean?
225 is the tyre's section width in millimetres, 45 is the sidewall height as a percentage of that width, R means radial construction and 17 is the rim diameter in inches. Here the sidewall is 225 × 0.45 = 101.25 mm.
How is the overall diameter worked out?
Diameter = rim size × 25.4 + 2 × sidewall height. For 225/45R17 that is 431.8 + 202.5 = 634.3 mm, or 24.97 inches. The circumference is π times the diameter.
How much diameter difference is acceptable when changing tyres?
Keeping it within a percent or two avoids big speedometer and gearing changes, and fitting a size your vehicle maker approves matters more than any rule of thumb. The check here shows the exact difference in percent.
Why is the real rolling distance a little shorter than the circumference?
A tyre carrying a load flattens slightly at the ground and wears down over its life, so it rolls a bit less than the unloaded figure. The calculator uses the nominal size from the code.